1. Field of the Invention
The invention relates to a catalytic converter for the catalytic treatment of exhaust gas. The catalytic converter is provided in particular to purify and/or to detoxify exhaust gas from an internal combustion engine, for example from a petrol combustion engine, by a catalytic treatment, i.e. to free the exhaust gas from pollutants by converting the latter by a chemical reaction. The internal combustion engine may, for example, belong to an automobile or other motor vehicle or be used for stationary operation--for example for driving an emergency power generator.
2. Description of the Prior Art
Catalytic converters known in practice for the catalytic purification of exhaust gas of a petrol combustion engine of an automobile have a casing with a lateral wall approximately circular or elliptical or oval in cross-section and with two end walls which are each provided in the center with a cylindrical connection and widen more or less conically from this to the lateral wall. One connection serves as an inlet and the other as an outlet. A catalyst element which has a number of passages parallel to the longitudinal axis is arranged in the casing. The catalyst element frequently has a carrier which consists of ceramic or steel and is coated with a catalytically active material containing at least one noble metal.
During operation of the known catalytic converter of the types described above, the exhaust gas fed to a catalytic converter flows, after passing the opening of the cylindrical inlet, into a cavity which is present between this inlet and the catalyst element and which is defined by the more or less conically widening end wall and possibly also by a short section of the lateral wall forming a circular or elliptical or oval cylinder. In this cavity, the exhaust gas forms approximately a jet which is directed at that end face of the catalyst element which faces the inlet. Introduction of the exhaust gas into the catalytic converter in this manner causes considerable turbulences and a large pressure loss. Furthermore, the jet mentioned results in the flow velocities and the flow rates in the catalyst element being inhomogeneous and being substantially greater in its central cross-sectional region aligned with the inlet opening than in the peripheral cross-sectional region. Since the flow path in the catalyst elements of the known catalytic converters is relatively long and furthermore a relatively large part of the exhaust gas flows at high velocity through the central cross-sectional region of the catalyst element, these likewise cause a large pressure loss in the known catalytic converters. The relatively large pressure loss produced between the inlet and the catalyst element and in the latter in the case of the known catalytic converters in turn results in a relatively large power loss of the internal combustion engine. In the case of catalyst elements having a volume corresponding approximately to the engine capacity, the total pressure loss due to the catalytic converter is, for example, of the order of 10 kPa, with the result that, for example, a power loss of about 2 kW to 3 kW can result in an internal combustion engine of an automobile of the medium power class.
The described inhomogeneity of the flow rate in the catalyst element furthermore has the disadvantage that the catalyst element is subjected to much greater stress in the central cross-sectional region than in the peripheral cross-sectional region. In order to achieve sufficient purification of the exhaust gas in spite of the inhomogeneous distribution of the flow over the cross-sectional area, the catalyst element must be provided with larger dimensions than would be necessary in the case of homogeneous flow distribution. As a result, the catalyst becomes much more expensive--in particular owing to the high price of the noble metal forming the catalytically active layer.
British Patent Disclosure 2 062 487 has already disclosed catalytic converters in which the exhaust gas is deflected between the orifice of the inlet and the exhaust air entry surface of the catalyst element. The casing of these catalytic converters has a cylindrical lateral wall and conical end walls which are adjacent to the ends of said side wall and taper away from said side wall. The inlet and the outlet of the casing each consist of a cylindrical pipe coaxial with the axis of the casing. A hollow cylindrical, gas-permeable catalyst element whose internal diameter is approximately equal to the external diameter of the pipe is arranged in the casing. In some of these catalytic converters, the pipe forming the inlet projects into the cavity enclosed by the annular catalyst element, is provided with holes in the region of this cavity and is closed by a terminating wall at that end surface of the catalyst element which faces the outlet. The exhaust gas flowing through the inlet into these catalytic converters during operation of said catalytic converters is backed up and deflected by the terminating wall and then flows into the catalyst element at the inner surface of said element. However, such backing up and deflection of the exhaust gas causes turbulences and a large pressure loss. In addition, a considerable axial pressure gradient, which in turn may cause a nonuniform distribution of the exhaust gas in the catalyst element, is produced in the cavity enclosed by the catalyst element. As already mentioned, a nonuniform distribution of the exhaust gas flowing through the catalyst element may increase the pressure losses produced in the catalyst element.
In the catalytic converter shown in the final Figure of British Patent Disclosure 2 062 487, the pipe forming the inlet enters the inner space of the casing at the thinner end of one of the conical end walls. An outer, annular cavity is present between the cylindrical lateral wall of the casing and the outer lateral surface of the annular catalyst element. In the inner space of the housing, a guide element is arranged on that end face of the catalyst element which faces the inlet. The guide element has a point projecting towards the orifice of the inlet and forms a conical guide surface which is inclined at 45.degree. to the casing axis. The guide element deflects the exhaust gas flowing through the inlet into the inner space of the casing during use of the catalytic converter outwards into the outer cavity. However, the exhaust gas flowing at relatively high velocity from the inlet into the inner space of the housing may become detached from the wall of the casing at the edge forming the orifice edge of the inlet, so that the flow tends to form turbulences there which may cause a considerable pressure loss. The formation of turbulences and the pressure loss are furthermore reinforced by the point of the guide element. As already explained, the pressure losses or pressure gradients produced in the catalytic converter cause losses of performance in the internal combustion engine connected to the catalytic converter.
Since the conical end walls of the casings of the catalytic converters disclosed in British Patent Disclosure 2 062 487 make an angle of only 35.degree. to 40.degree. with the axis of the relevant casing, the casings are relatively long compared with the axial dimensions of the catalyst elements. This may be disadvantageous in particular in the case of catalytic converters for installation in exhaust systems of automobiles or other motor vehicles, since there is often little space available there.